(1) Field of the Invention
The present invention relates to a water-repellent nonwoven fabric prepared by melt-blowing. More particularly, the present invention relates to a nonwoven fabric which has an excellent air permeability and oil permeability, and a non-permeability to water.
The present invention also relates to an oil-water separating filter and an oil-water separating method, using the water-repellent woven fabric.
(2) Description of the Related Art
Most water-proof fabrics are prepared by coating the surface of a woven or knitted fabric with a resin coating. In the fabrics prepared according to this method, in order to maintain sufficient water-proofness, the air permeability must be sacrified. As means for solving this problem, Japanese Unexamined Pat. Publication No. 56-26,076 proposes a method fcr making a moisture permeable coated fabric, in which fine pores having a size of the micron order and being intercommunicating with each other are formed in a coating film formed of a synthetic polymer composed mainly of polyurethane. In this method, however, the number of pores that can be formed in the unit area is limited, and in order to obtain a sufficient water-proofness, the air permeability must be controlled to below 0.5 cc/cm.sup.2 .multidot.sec, and hence, a satisfactory air permeability cannot be obtained.
As the conventional oil-water separating technique, there can be mentioned a gravity separation method, a centrifugal separation method, an oil collecting method, a membrane separation method, a chemical flocculation method, a magnetic separation method, and a droplet-coalescing separation method. However, these methods have problems in that complete separation cannot be attained by a single step and or the separation cost is very high. As one separation method in which separation can be easily accomplished by a single step at a relatively low cost, an ultrafiltration method using a polymeric porous membrane is adopted in various fields. However, since the rate of liquid permeation is low, the amount of the liquid treated per unit time is small, and in order to avoid this disadvantage, the size of the separation apparatus must be increased. A method using a nonwoven fabric filter or filter paper having a high filtration rate is adopted, but this method still has problems in that the separation of an oil is incomplete or clogging readily occurs. Moreover, when a filter paper is used, the pulp is swollen with water to cause a reduction of the strength and a reduction of the permeation rate. As the droplet-coalescing separation method, there is known a method in which a fiber assembly coalescer composed of nylon, rayon, glass, or stainless steel fiber is used for forming coalescent oil droplets. This method exerts a high droplet-coalescing separation effect for relatively large oil droplets (having a diameter larger than 5 .mu.m). However, for fine stable oil droplets having a size of about 0.1 to about 5 .mu.m, or when the oil concentration is too high, coalescing of the oil droplets becomes impossible. Moreover, even for oil droplets having a size larger than 5 .mu.m, it is difficult to form properly coalescent droplets by a single stage of the coalescer, and therefore, a plurality of stages of coalescers must be used. As means for solving this problem, Japanese Examined Patent Publication No. 55-45,242 proposes an element composed of an assembly of a fiber having an affinity with fine oil droplets, in which the void ratio is reduced to coarsen the fine droplets. However, even according to this proposal, it is difficult to form properly coalescent and separate ultra-fine stable oil droplets having a size of about 0.1 to about 1 .mu.m, and since the void ratio is small, the amount of the liquid treated per unit area is small.
Water diffused from the human body ordinarily includes water evaporated from the human body and sweat formed for adjusting the body temperature during motion. The rate of the former evaporation is 350 to 600 g/m.sup.2 .multidot.24 hours at a comfortable temperature in the quiet state, and upon violent motion, the rate of the latter evaporation is said to be 2,500 g/m.sup.2 .multidot.24 hours. A cloth, diaper or shoe formed of the conventional water-proof fabric inhibits diffusion of water from the body to the outside of the fabric, as pointed out hereinbefore, and the humidity is increased within the fabric to impart a stuffy, and unpleasant feeling. In the conventional moisture-permeable water-proof fabric, the water-proofness of at least 600 mmH.sub.2 O as expressed as the water pressure resistance and the moisture permeability of at least 5,000 g/m.sup.2 .multidot.24 hours are simultaneously attained, but a sufficient air permeability, that is, an air permeability of at least 1.0 cc/cm.sup.2 .multidot.sec, is not attained. Ordinarily, the air permeability is incorrectly regarded as the same property as the moisture permeability, but the levels required for both properties are vastly different and the two properties should be clearly discriminated from each other. The moisture permeation rate of 5,000 g/m.sup.2 .multidot.24 hours is converted to a molecule flow rate of 3.215.times.10.sup.-7 mole/cm.sup.2 .multidot.sec, and the air permeation rate of 1.0 cc/cm.sup.2 .multidot.sec is converted to a molecule flow rate of 4.16.times.10.sup.-5 mole/cm.sup.2 .multidot.sec. When these values and the conditions for measuring the moisture permeation rate and air permeation rate are substituted in the Hagen-Poiseuille equation determining the permeation coefficient, it is found that a permeation coefficient of 1.313.times.10.sup.-9 cm is necessary for attaining the moisture permeation rate of 5,000 g/m.sup.2 .multidot.24 hours, and a permeation coefficient of 5.766.times.10.sup.-7 cm is necessary for attaining the air permeation rate of 1.0 cc/cm.sup.2 .multidot.sec (examples of the calculation of the permeation coefficient will be described hereinafter). Accordingly, it is seen that the required level of air permeability is 439 times as high as the required level of moisture permeability. Therefore, if the air permeability is sufficient, the level of the moisture permeability is more than 439 times as high as the level of the conventional water-proof fabric, and it is considered that a fabric satisfying this requirement is comfortable and does not give a stuffy feeling because diffused water is sufficiently discharged outside the fabric. In the case of clothing, especially a diaper, a sufficient water-proofness should be attained under compression by the body weight of a baby, and the water pressure resistance index should be ordinarily 300 to 400 mmH.sub.2 O.